The Sci-Fi TV channel has posted a large collection of first-rate science fiction short stories, free for the clicking. Includes some classic stories by prominent authors. Such as Norman Spinrad's "Carcinoma Angels". I read it first when I was twelve; it took my breath away. And it indicated that I had no future as a fiction writer as long as I would have to compete with stories like this.
Thirty-six years later it holds up superbly.
To me, the key issue for a good short story is the ending. It's not all that hard to give the main character an interesting problem. But the trick is how the author resolves the problem. Every aspiring author should see how Spinrad did it here.